3D Distance is the distance from a point to a surface represented by XYZ coordinates.Normal Deviation (dN) is the simple 3D distance from a point to a surface along the normal vector.

For theoretically infinite features, such as planes or cylinders, the 3D Distance is equal to the Normal Deviation.

For features with trimmed edges, such as surfaces, the 3D Distance is equal to the Normal Deviation if the projected point is in the middle of the surface. If the point being compared is off to the side, falls on the edge, or is otherwise outside the bounds of the surface, the 3D Distance will likely be larger than the Normal Deviation.

For vector points with a fixed nominal point, the 3D Distance will likely be larger than the Normal Deviation if the actual falls outside the zone.